Allen’s Test

Allen’s Test is a test used in medicine prior to arterial blood gas collection in order to determine normal patency of the ulnar artery.

Anatomy of Allen’s Test

The anatomies evaluated in Allen’s Test include both the ulnar and radial arteries. In normal anatomical and physiological function the hand is supplied with blood (perfusion) through the ulnar and radial arteries. If one is occluded, the other provides blood to the hand and maintains perfusion. In a very small percentage of the population, only the radial or only the ulnar artery is functioning, and therefore they lack the ability to have dual blood supply to the hand.

How do you Perform an Allen’s Test?

To perform an Allen’s test:

1. The patient’s hand should be elevated above his or her heart;
2. The patient’s should be asked to make a fist;
3. Pressure should be applied to both the radial and the ulnar artery until distal blood flow is occluded;
4. While maintaining the elevated hand position, the patient should then open the hand. The hand should appear pale and have limited capillary refils;
5. The ulnar arterial pressure should be released (while maintaining enough pressure to occlude the radial artery).
6. The hand should return to normal colour within 5-7 seconds.

If the patient’s hand returns to normal colour within 3-7 seconds the Allen’s test is said to be negative and the patient has normal dual blood supply. If the patient’s hand returns to normal after 7 seconds, the Allen’s test is said to be negative and the patient does not have dual blood supply to the hand (or if he or she does, it is very small).

What is the Clinical Significance of Allen’s Test?

When the Allens test is positive (meaning that the patient does not have dual blood supply to the hand), he or she will often have a negative result for the other hand. Therefore, to reduce the risk of ischemia to that hand, it is important to perform the cannulation or arterial blood gas collection from the hand with dual blood supply.

What is the clinical significance of Allens test in Paramedics?

The clinical benefits of Allens test in paramedics is only relevant when you are about to attempt to cannulate near the radial or ulnar artery. It is uncommon as a paramedic to need to cannulate the veins in this region; however, in some IV drug users, or people who have been on steroids for many years, that most of their veins are damaged, and consequently these are the only veins left open to use. In this circumstance, it is important to perform an Allens test before hand to ensure that you do not accidentally damage the radial or ulnar artery on the one hand without dual blood supply.